1. Field of the Invention
This invention relates to an electron-emitting device that is free from degradation due to long use and the undesired phenomenon of electric discharge under a voltage applied thereto and can emit electrons stably and efficiently for a long time. It also relates to an electron source and an image forming apparatus such as a display apparatus or an exposure apparatus comprising such devices as well as a method of manufacturing the same.
2. Related Background Art
There have been known two types of electron-emitting device; the thermionic cathode type and the cold cathode type. Of these, the cold cathode emission type refers to devices including field emission type (hereinafter referred to as the FE type) devices, metal/insulation layer/metal type (hereinafter referred to as the MIM type) electron-emitting devices and surface conduction electron-emitting devices. Examples of FE type device include those proposed by W. P. Dyke & W. W. Dolan, "Field emission", Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, "PHYSICAL Properties of thin-film field emission cathodes with molybdenum cones", J. Appl. Phys., 47, 5284 (1976).
Examples of MIM devices are disclosed in papers including C. A. Mead, "The tunnel-emission amplifier", J. Appl. Phys., 32, 646 (1961).
Examples of surface conduction electron-emitting devices include one proposed by M. I. Elinson, Radio Eng. Electron Phys., 10 (1965).
A surface conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elinson proposes the use of an SnO.sub.2 thin film for a device of this type, the use of an Au thin film is proposed in G. Dittmer: "Thin Solid Films", 9, 317 (1972) whereas the use of In.sub.2 O.sub.3 /SnO.sub.2 and of carbon thin film is discussed respectively in [M. Hartwell and C. G. Fonstad: "IEEE Trans. ED Conf.", 519 (1975)] and [H. Araki et al.: "Vacuum", Vol. 26, No. 1, p. 22 1983.
FIG. 33 of the accompanying drawings schematically illustrates a typical surface conduction electron-emitting device proposed by M. Hartwell. In FIG. 33, reference numeral 1 denotes a substrate. Reference numeral 4 denotes an electroconductive thin film normally prepared by producing an H-shaped thin metal oxide film by means of sputtering, part of which eventually makes an electron-emitting region 5 when it is subjected to an electrically energizing process referred to as "energization forming" as described hereinafter. In FIG. 33, the thin horizontal area of the metal oxide film separating a pair of device electrodes has a length L of 0.5 to 1 mm and a width W of 0.1 mm.
Conventionally, an electron emitting region 5 is produced in a surface conduction electron-emitting device by subjecting the electroconductive thin film 4 of the device to an electrically energizing preliminary process, which is referred to as "energization forming". In the energization forming process, a constant DC voltage or a slowly rising DC voltage that rises typically at a rate of 1V/min. is applied to given opposite ends of the electroconductive thin film 4 to partly destroy, deform or transform the film and produce an electron-emitting region 5 which is electrically highly resistive. Thus, the electron-emitting region 5 is part of the electroconductive thin film 4 that typically contains a gap or gaps therein so that electrons may be emitted from the gap.
After the energization forming process, the electron-emitting device is subjected to an "activation" process, where a film (carbon film) of carbon and/or one or more than one carbon compounds is formed in the vicinity of the gap of the electron source in order to improve the electron-emitting performance of the device. The process is normally carried out by applying a pulse voltage to the device in an atmosphere that contains one or more than one organic substances so that carbon and/or one or more than one carbon compounds may be deposited in the vicinity of the electron-emitting region. Note that a deposited carbon film is found mainly on the anode side of the electroconductive thin film and only poorly, if any, on the cathode side. In some cases, a "stabilization" process may be carried out on the electron-emitting device in order to prevent carbon and/or one or more than one carbon compounds from being excessively deposited and the device may show a stabilized performance in the operation of electron emission. In the stabilization process, any organic substances that have been adsorbed in the peripheral areas of the device and those that are remaining in the atmosphere are removed.
For a surface conduction electron-emitting device to operate satisfactorily in practical applications, it has to meet a number of requirements including that it needs to show a large emission current Ie and a high electron emission efficiency .eta. (=Ie/If, where If is the current that flows between the two device electrodes, which is referred to as device current), that it must operate stably for electron emission after a long use and that no electric discharge phenomenon should be observed on it if a voltage is applied to the device (between the two device electrodes and between the device and an anode).
While the performance of an electron-emitting device is affected by a number of factors, the inventors of the present invention has discovered that the performance is strongly correlated with the shape and the distribution of the carbon film formed on the electron-emitting gap and its vicinity in the activation process as well as the conditions under which the activation process is carried out.